Field of the Invention
This invention relates to pressure relief valves or the like and more particularly to such valves which are prone to self-oscillation.
Pressure relief valves generally function to cause the valve poppet to open either against or in the direction of fluid flow from the tank or other container which is subject to a varying pressure, however the instant that the valve poppet opens within the pressure relief line, the release of the fluid under pressure tends to reduce the pressure within the container subject to pressure relief, causing the poppet valve to tend to reverse its direction, that is, shift back towards closed position, and in many cases, high velocity oscillation and chattering of the poppet results, particularly where the pressure relief valves are mounted within a valve casing which is essentially remote from the tank or container with which it functions.
Prior pressure relief valves generally oscillate due to a reverse flow-energy poppet control system. Attempts to achieve a chatter-proof relief valve have been made, but these are particularly inapplicable where the pressure relief valve which opens to the interior of a tank above an accumulated liquid requires that the pressure relief valve be connected by means of a vent line which extends for a substantial distance through the liquid being stored and provided with a relief valve which operates at a point quite remote from the source of pressure. Under these installation conditions, there is of necessity required a very long standpipe vent line leading to the pressure relief valve and preferably a tank top sensor for actuation of the control device responsive to pressure variation for, in turn, opening the valve poppet at the bottom of the standpipe vent line.
Conventional pressure relief valves normally employed for tank top pressure relief are not satisfactory for mounting to the bottom of the tank without extensive alterations and are not satisfactory in operation when shifted to that position. The problem is that conventional main valve design is not adaptable to the inherent time delay due to the extensive length of the standpipe vent line leading to the valve, the valve is prone to self-oscillation unless pilot feedback can be almost instantaneous, and the valve will amplify any "organ pipe" oscillation occurring in the standpipe as well as its own starting or initial opening instability, mentioned previously.
The present invention is directed to the utilization of a primary pilot relief valve for controlling the relief pressure level over the entire flow range from main valve poppet cracking to full flow, and to a main valve actuator piston which is isolated from the main relief valve chamber and so is unaffected by and can damp out pressure oscillations and perturbations occurring within the valve body due to initiation of flow past the main valve.
Further, in many applications, the function of the pressure relief valve is to insure that it shall fail closed. With a single pilot system, there is no way to assure that any pilot controlled pressure system cannot fail closed or open or even with some complex dual systems.
The present invention has for its further object a control system employing multiple pilot valves in which dual failures are required to fail the valve open, while permitting a single failure to fail it closed.
It is a further object of this invention to provide a multiple pilot valve operated pressure relief valve which insures upon failure of any part of the dual pilot system that the valve will surely fail closed.
It is yet another object of this invention to provide a multiple pilot valve control pressure relief system in which dual pilot valve failures are required to fail the main poppet valve open or closed.